Astroparticle Physics

6.3 Gamma Astronomy 119Future space-based gamma-ray telescopes like INTE-GRAL 9 (15 keV–10 MeV) and GLAST 10 (30 MeV–300GeV) will certainly supplement the results from earlier γ -ray missions.In the TeV range γ -ray point sources have been discoveredwith the air Cherenkov technique. Recently, a supernovaremnant as a source of high-energy photons has beenobserved by the HESS 11 experiment. The γ -ray spectrumof this object, SNR RX J1713.7-3946 in the galactic plane7 kpc off the galactic center, near the solar system (distance1 kpc) can best be explained by the assumption that the pho- hadron acceleratortons with energies near 10 TeV are produced by π 0 decays,i.e., this source is a good candidate for a hadron accelerator.The imaging air Cherenkov telescope MAGIC underconstruction on the Canary Island La Palma with its 17 mdiameter mirror will soon compete with the HESS telescopein Namibia. Apart from galactic sources (Crab Nebula) extragalacticobjects emitting TeV photons (Markarian 421, TeV photonsMarkarian 501, and M87) have also been unambiguouslyidentified. Markarian 421 is an elliptic galaxy with a highlyvariable galactic nucleus. The luminosity of Markarian 421in the light of TeV photons would be 10 10 times higher thanthat of the Crab Nebula if isotropic emission were assumed.One generally believes that this galaxy is powered by a massiveblack hole which emits jets of relativistic particles fromits poles. It is conceivable that this approximately 400 millionlight-years distant galaxy beams the high-energy particlejets – and thereby also the photon beam – exactly intothe direction of Earth.The highest γ energies from cosmic sources have been highest γ energiesrecorded by earthbound air-shower experiments, but also byair Cherenkov telescopes. It has become common practiceto consider the Crab Nebula, which emits photons with energiesup to 100 TeV, as a standard candle. γ -ray sourcesfound in this energy regime are mostly characterized byan extreme variability. In this context the X-ray sourceCygnus X3 plays a special rôle. In the eighties γ rays fromthis source with energies up to 10 16 eV (10 000 TeV) wereclaimed to be seen. These high-energy γ rays appeared toshow the same variability (period 4.8 hours) as the X rays9 INTEGRAL – INTErnational Gamma-Ray Astrophysics Laboratory10 GLAST – Gamma Ray Large Area Telescope11 HESS – High Energy Stereoscopic System